Thermoplastics such as PP, PE, etc., are typically coloured using pigment concentrates. The pigment concentrates are often referred to as masterbatches or compounds. These concentrates are prepared by mixing ingredients together and subjecting them to any of the processes commonly used for dispersing particulate solids in a thermoplastic polymer. Compounding or mixing in a twin-screw extruder is one of these processes. Pigment concentrations in masterbatches may contain up to 70% of pigment, and optionally other additives. Other additives may include waxes, dispersants, lubricants and UV stabilizers.
Production of masterbatches typically utilises a pigment dispersed in thermoplastic polymer, i.e., fine particles with limited amounts of aggregates. However, aggregates are known to form from the pigments in the production of masterbatches. The presence of aggregates tends to result in filter blocking of the extruder.
In addition, particular requirements are important for finished articles containing the thermoplastic polymer. The thermoplastic polymer may be in the form of a solid article, a film or fiber. In solid articles, acceptable dispersion of the pigment is necessary in order to maximise colour development, tinctorial strength and reduced speck levels. For films, incomplete dispersion of the pigment may lead to cracking, unwanted light scattering effects and specks. In fibres, incomplete dispersion of the pigment may result in fiber breakage.
U.S. Pat. No. 4,797,400 discloses the use of a polymer containing a carboxylate functionality, e.g., poly(12-hydroxystearic acid) or polyricinoleic acid, as a dispersant in the presence of a particulate solid, a lubricant and a thermoplastic polymer.
Waxes are also known to disperse particulate solids in thermoplastic polymers, see U.S. Patent Application 2002/0151639.